1. Field of the Invention
The present invention relates generally to an electrochemical device, and more particularly to improvements in an electrochemical device of laminar structure which comprises an electrochemical cell having a planar solid electrolyte body.
2. Related Art Statement
There are known various electrochemical devices using a solid electrolyte material, for example as oxygen sensors to detect the oxygen concentration in exhaust gases which are emitted from an internal combustion engine of an automotive vehicle. A typical example of such oxygen sensors comprises a body of oxygen-ion conductive solid electrolyte such as zirconia ceramics. This oxygen sensor operates to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the art of electrochemical devices are detectors such as sensing and pumping elements for detecting hydrogen, nitrogen, carbon dioxide, etc. In such type of electrochemical devices, solid electrolyte materials have been used generally in the form of a tubular body which has an elongate bore closed at its one end. Recently, however, there has been an increasing effort to replace the tubular solid electrolyte body with a planar solid electrolyte body, as disclosed in U.S. Pat. Nos. 4,334,974; 4,282,080; and 4,300,990, in view of relatively low productivity and relatively high cost of manufacture of the solid electrolyte bodies of tubular configuration, and from the standpoint of easy assembling of parts with a planar solid electrolyte body. In the case where such planar solid electrolyte bodies are used, suitable electrodes are formed on the surfaces of the solid electrolyte layers, such that the solid electrolyte layers and other layers are stacked on each other into a laminar structure which constitute an electrochemical cell or element.
Generally, an electrochemical cell of the laminar structure indicated above is constituted by a planar solid electrolyte body and at least one pair of electrodes. An electrochemical sensing element of an electrochemical device comprises an electrochemical pumping cell and an electrochemical sensing cell both of which have the laminar structure as described just above. These pumping and sensing cells are superposed on each other, so that the electrochemical sensing element itself has a laminar structure. The laminar sensing element is provided with an internal thin flat measurement-gas space which is formed in communication with an external space in which an exernal measurement gas exists. This internal flat space has a thickness which is determined to provide a predetermined diffusion resistance to the measurement gas. One of the two electrodes of each of the pumping and sensing cells is disposed so that it is exposed to the measurement gas in the flat measurement-gas space. The electrochemical pumping cell performs a pumping function to control the concentration of a given component in the measurement gas within the measurement-gas space. In the meantime, the electrochemical sensing cell serves to measure an electromotive force which is induced between its two electrodes, due to a difference in concentration of the measurement component between the controlled measurement gas within the measurement-gas space and a suitable reference gas.
The inventors of the present invention studied the aforementioned laminar electrochemical element wherein one of the two electrodes of each electrochemical cell is exposed to the flat measurement-gas space. The study revealed an inconvenience that when the measurement gas is an ambient air and the air in the measurement-gas space is controlled by the pumping cell, there exists a difference in concentration of a measurement component between an atmosphere adjacent to the pumping electrode and an atmosphere adjacent to the measuring electrode of the sensing cell, due to a resistance of diffusion in the direction of thickness or depth of the measurement gas space. Consequently, the accuracy of detection is not satisfactory. Further, the electromotive force induced between the electrodes of the sensing cell tends to be extremely small.
An electrochemical element as discussed above is usually used for measuring exhaust gases which are emitted for example by internal combustion engines. If the ambient air is used as a measurement gas, however, a pumping action of the pumping cell of the electrochemical element will not cause the atmosphere within the flat measurement-gas space to become like a rich-burned exhaust gas which is produced as a result of combustion of an air-fuel mixture having an air/fuel ratio lower than the stoichiometric point. Described more specifically, when exhaust gases are introduced as a measurement gas into the flat measurement-gas space, CO.sub.2, H.sub.2 O and other gases which are contained in the exhaust gases as a result of combustion of an air-fuel mixture are reduced by electrode reaction of the pumping cell, whereby reductive gases are produced within the measurement-gas space. These reductive gases enable the atmosphere in a portion or a major portion of the measurement-gas space to have a relatively low concentration of oxygen. If ambient air is introduced into the measurement-gas space, the oxygen concentration can be held relatively low only in a portion of the measurement-gas space adjacent to the pumping electrode, since nitrogen is a major component of the ambient air, other than oxygen.
For the above reason, it is impossible to use ambient air as a test gas for testing the electrochemical element, in an industrial production of electrochemical devices, to check for the diffusion resistance of its measurement-gas space, that is, the limiting current of the pumping cell. Therefore, it is assumed that the testing of the electrochemical element requires a specially designed complicated device for producing a test gas in which the electrochemical element is checked for its pumping performance.
In addition, even after the electrochemical device is installed as an oxygen sensor on an automotive vehicle, it is necessary to conduct a periodic test of the electrochemical element to check for deterioration of its pumping capability or performance during its service, and to adjust the oxygen sensor as needed, to permit the sensor to perform an accurate sensing operation. Since ambient air can not be used as a test gas as previously indicated, it is almost impossible to test the oxygen sensor while it is installed on the vehicle.